The toxicity of metals has been well documented, and releasing effluents containing metals into the environment is now strictly controlled by environmental protection laws. For instance, the effluent limitations on copper are as low as about 0.0017 pounds of copper per 1,000 pounds of effluent (about 1.7 parts per million). 40 C.F.R. .sctn. 421.52. The electronics industry generates large volumes of used circuit-board etching solution containing high levels of copper ions. This effluent cannot be discharged directly into the environment without first being subjected to expensive procedures to reduce the effluent copper concentration.
Some processes are known for removing metal ions from solution. For instance, Spedden et al.'s U.S. Pat. No. 3,634,071 concerns a process for accelerating the precipitation of copper from solution. According to Spedden's process, a solution containing copper ions must be treated with sulfur dioxide. The purpose of the sulfur dioxide is to produce hydronium ions (H.sup.+) by reacting with water to form sulfurous acid (H.sub.2 SO.sub.3). This compensates for the consumption of hydronium ions that occurs during the reduction of copper ions. Spedden teaches that using sulfuric acid as the sole source of hydronium ions is unacceptable in a process for recovering copper ions from solution.
Guess' U.S. Pat. No. 5,122,279 also describes a process for removing dissolved metals from water. The process comprises treating solutions containing heavy metals with ferrous dithionate to form a complex which precipitates from solution. Guess attempted a number of copper recovery trials to compare his process to other processes for recovering copper. Example I of Guess discusses treating a feed solution with steel wool. Guess indicates that some removal of copper was visually evident as the steel-wool mass gradually turned from steel color to copper-colored fibers. Atomic absorption spectrometry of the treated solution indicated that only 28 percent of the copper in the solution was collected on the steel wool.
Example VI of Guess discusses removing copper from solution using iron particles. A liter of a solution having a pH of about 9 and containing copper ions was treated with an equimolar quantity of iron particles. A sample of this solution was withdrawn for filtration after a coating of metallic copper was deposited on the surface of the iron particles. Analysis of the sample determined that approximately 87 percent of the copper remained in solution. Hence, Example VI of the Guess patent teaches that only about 15 percent of the copper ions in solution can be recovered as copper metal by treating the solution with iron powders. A 15 percent conversion is obviously unacceptable when the copper concentration in an effluent can be no greater than about 2 ppm.
Guess also discusses U.K. Patent Application GB 125828 A. This application describes a process involving contacting a solution containing copper ions with steel wool. According to Guess, the problems associated with this method include: (1) an uneconomically low conversion of copper ion to copper metal; (2) a high cost associated with steel wool; and (3) a high labor cost associated with handling the materials.
Despite ongoing investigations, a process for efficiently and inexpensively recovering metals from solutions containing ions of the metal still has not been developed. Although it is known to treat solutions with iron to precipitate low levels of copper, this reaction has not been effectively tailored for recovering copper ions from used circuit-board etching solutions. Moreover, the patents discussed above teach that simply using iron to recover copper metal is an inefficient and unacceptable method for recovering copper from solutions containing copper ions.